Electrical power supply devices for apparatuses are in common use. It can be chargers for mobile telephones, personal digital assistants, laptop computers etc., or direct current (DC) power supply devices for electronics such as wireless local access network nodes, computer accessories, consumer electronics, etc. The power supply devices are normally arranged to provide a DC output. The power supply devices can be connected to alternating current mains for down-conversion and rectifying of voltage, or be connected to a DC output, for example in a car, for down-conversion or up-conversion of the voltage to a desired level. A problem that arises from these supply devices is that energy is consumed, although at a low power level, when the supply device is left plugged into the input supply, but as the supply devices often are left there for long time, a non-negligible amount of energy is consumed. For example, a mobile phone charger is permanently connected to mains, and used for charging a mobile phone during a few hours a week, but will consume stand-by power the remaining time. It is therefore a problem how to reduce energy consumption in an electrical power supply device.